A typical, conventional gas turbine engine includes one or more multi-stage compressors, a combustor, and one or more turbines. An annular working medium flowpath extends through the compressors, combustor and turbines. A drive shaft connects each turbine to an associated compressor. At least one of the drive shafts is supported from the nonrotatable structure of the engine by bearings, which are enclosed in a bearing compartment. A lubrication system introduces oil or other lubricant into the bearing compartment to lubricate and cool the bearings, and also reconditions the oil for re-use. Bearing compartment seals help prevent oil leakage out of the compartment by segregating the interior of the compartment from its local environment.
It is common practice to buffer the bearing compartment seals to enhance their ability to resist oil leakage. Buffering refers to directing pressurized air to the vicinity of the seals, outside the bearing compartment. The buffer air is usually pressurized air extracted from the working medium flowpath, specifically from the compressor flowpath. The pressure of the buffer air exceeds the prevailing pressure inside the bearing compartment. This results in a favorable, positive pressure difference across the compartment seals. Because of the positive pressure difference, buffer air flows into the compartment, helping to confine the oil.
The designer of the lubrication system must select a satisfactory location (i.e. stage) in the compressor from which to extract the buffer air. Air extracted from a higher pressure stage of the compressor has the advantage of establishing a sizeable positive pressure difference across the bearing compartment seals. This allows a large quantity of buffer air to flow into the compartment, which forms a highly effective barrier against oil leakage. However the large quantity of pressurized air can also cause the oil in the bearing compartment to foam, which compromises its lubricating and heat transfer properties. In addition, the high pressure air has a correspondingly high temperature. Introducing a large quantity of high temperature air into the bearing compartment could ignite and sustain a fire inside the compartment.
The designer can guard against oil foaming and mitigate the risk of fire by choosing to extract the buffer air from a lower pressure stage of the compressor. When the engine operates at high power, this lower stage buffer air will have enough pressure to maintain a positive pressure difference across the bearing compartment seals. However when the engine operates at lower power settings, including idle, the pressure throughout the compressor, and therefore the pressure of the lower stage buffer air, will be significantly reduced. In order to guarantee that the lower stage buffer air doesn't compromise the positive pressure difference across the bearing compartment seals at low engine power, it may be necessary to operate the engine at an undesirably high idle setting. In other words, the need to always maintain a positive pressure difference across the bearing compartment seals (in order to prevent oil leakage), in combination with the use of low stage buffer air, may demand an undesirably high idle power setting. A high idle setting is undesirable because it promotes excessive engine fuel consumption. Moreover, if the engine is used as an aircraft powerplant, a high idle setting can make the aircraft more difficult to handle during descent and taxiing maneuvers. Of course, the designer can overcome these problems by choosing to extract the buffer air from a higher pressure stage of the compressor, but doing so re-introduces the foaming and fire risk sought to be avoided by selecting a lower pressure air source in the first place.
The task of selecting an appropriate buffer air source without requiring an undesirably high idle power setting is further complicated by the presence of a deoiler in the lubrication system. The buffer air and the lubricating oil cross-contaminate each other in the bearing compartment. The deoiler receives the oil-contaminated air, now referred to as breather air, and separates the oil from the air so that the oil can be re-used. The lubrication system then vents the decontaminated breather air into the atmosphere. The deoiler is a desirable component because the contaminated air would contribute to air pollution if it were discharged, untreated, directly into the atmosphere. Moreover, if the oil were vented overboard with the breather air, it would be necessary to carry a larger supply of oil on the aircraft thereby increasing aircraft weight and consuming precious space.
Unfortunately, a conventional lubrication system deoiler also pressurizes the bearing compartment by restricting the flow of breather air out of the compartment. In other words, the presence of a conventional deoiler causes the bearing compartment pressure to be higher than it would be if the bearing compartment were vented directly to the atmosphere. This elevated compartment pressure requires that the buffer air pressure also be correspondingly elevated in order to ensure a positive pressure difference across the bearing compartment seals at all ambient conditions and engine power settings, including idle. This elevated compartment pressure can be beneficial at high engine power settings because it helps prevent excessive infiltration of hot, high pressure buffer air into the compartment. But at lower power settings, the elevated compartment pressure, in combination with the use of lower stage buffer air, can require an unsatisfactorily high idle power setting to ensure a positive pressure difference across the bearing compartment seals.